5-fluorouracil (5-FU) was first introduced in 1957, and still remains an essential part of the treatment of a wide range of solid tumors such as breast tumors, tumors of head and neck and gastrointestinal tumors.
5-FU is an example of a rationally designed anticancer agent. Observations of utilization of uracil in rat liver tumors indicated that the utilization of this nucleobase (there are four nucleobases in the nucleic acid of RNA) [Berg J M; Tymoczko J L; Stryer L (2002). Biochemistry (5th ed.), WH Freeman and Company. pp. 118-19, 781-808. ISBN 0-7167-4684-0. OCLC 179705944] was more pronounced in the tumors than in non-malignant tissue. This implicated that the enzymatic pathways for uracil utilization differs between malignant and normal cells [Rutman R J et al. Studies in 2-acetylaminofluorene carcinogenesis. III. The utilization of uracil 2-14C by preneoplastic rat liver and rat hepatoma. Cancer Res 1954; 14: 119-123]. 5-FU was then synthesized as an antimetabolic agent [Heidelberger C et al. Fluorinated pyrimidines, a new class of tumor-inhibitory compounds. Nature 1967; 179: 663-666]. In 5-FU, the hydrogen atom in position 5 of uracil is replaced by the similar sized atom of fluorine, and 5-FU was designed to occupy the active sites of enzymes, blocking the metabolism of malignant cells.
The overall response rate of 5-FU alone is quite limited, reaching levels of 10-15% [Johnston P. G., Kaye S. Capcetabine; a novel agent for the treatment of solid tumors. Anticancer Drugs 2001, 12: 639-646] and modulation strategies to increase the anticancer activity of 5-FU have been developed. One of the most widely used strategies is a co-administration of Leucovorin, the calcium salt of folinic acid. Leucovorin (LV) acts as a stabiliser of the ternary complex, a structure formed by 1) 5,10-methylene tetrahydrofolate, the active metabolite of LV, of 2) FdUMP, the 5-FU active metabolite and of 3) Thymidylate synthase. This ternary complex inhibits the enzyme thymidylate synthase, an enzyme necessary for DNA synhesis [Longley D. B. et al. 5-Fluorouracil. Mechanisms of action and clinical strategies, Nat Rev Cancer. 2003 May; 3(5):330-8. Review]. By adding LV to 5-FU the overall response rates increased to over 20% [Longley D. B. et al. 2003 ibid.].
Breast cancer is the most frequently diagnosed cancer and the leading cause of cancer-related death among females worldwide (Breast Cancer, http://www.cancerresearchuk.org/cancer-info/cancerstats/world/breast-cancer-world/.) Despite the gains in early detection, up to five percent of women diagnosed with breast cancer in the United States have metastatic disease at the time of first presentation. In addition, up to 30 percent of women with early-stage, non-metastatic breast cancer at diagnosis will develop distant metastatic disease [Early Breast Cancer Trialists' Collaborative Group (EBCTCG). Effects of chemotherapy and hormonal therapy for early breast cancer on recurrence and 15-year survival: an overview of the randomised trials. Lancet 2005; 365:1687]. Although metastatic breast cancer is not curable, meaningful improvements in survival have been seen, coincident with the introduction of newer systemic therapies see [Chia S. K., Speers C. H., D'yachkova Y. et al. The impact of new chemotherapeutic and hormone agents on survival in a population-based cohort of women with metastatic breast cancer. Cancer 2007; 110:973] and [Gennari A., Conte P., Rosso R. et al. Survival of metastatic breast carcinoma patients over a 20-year period: a retrospective analysis based on individual patient data from six consecutive studies. Cancer 2005; 104:1742] and [Dafni U., Grimani I., Xyrafas A. et al. Fifteen-year trends in metastatic breast cancer survival in Greece. Breast Cancer Res Treat 2010; 119:621].
The goals of treatment of metastatic breast cancer are to prolong survival and improve quality of life by reducing cancer-related symptoms. Cytotoxic chemotherapy (including the use of 5-FU) is particularly used in patients with hormone receptor-negative patients, patients with symptomatic hormone-receptor and a rapid disease progression or a large tumor burden involving visceral organs [Wilcken N., Hornbuckle J., Ghersi D.; Chemotherapy alone versus endocrine therapy alone for metastatic breast cancer. Cochrane Database Syst Rev 2003; :CD002747]. 5-FU is usually combined with cyclophosphamide and methotrexate (CMF). The reponse rate is around 20% and the OS around 20 months [Stockler M. R., Harvey V. J., Francis P. A. et al. Capecitabine versus classical cyclophosphamide, methotrexate, and fluorouracil as first-line chemotherapy for advanced breast cancer. J Clin Oncol 2011; 29:4498].
5-FU is also used for the treatment of advanced and recurring head and neck squamous cell cancer. The prognosis in this patient group is generally poor with a median survival time in most studies of 6-9 months. 5-FU is mainly used in combination therapies with platinum compounds. Response rates are around 30% but the survival time remains low, around 6 months see [Clavel M., Vermorken J. B., Cognetti F. et al. Randomized comparison of cisplatin, methotrexate, bleomycin and vincristine (CABO) versus cisplatin and 5-fluorouracil (CF) versus cisplatin (C) in recurrent or metastatic squamous cell carcinoma of the head and neck. A phase III study of the EORTC Head and Neck Cancer Cooperative Group. Ann Oncol 1994; 5:521] and [Forastiere A. A., Metch B., Schuller D. E. et al. Randomized comparison of cisplatin plus fluorouracil and carboplatin plus 5-fluorouracil versus methotrexate in advanced squamous-cell carcinoma of the head and neck: a Southwest Oncology Group study. J Clin Oncol 1992; 10:1245].
But it is among the gastrointestinal tumors where the 5-FU based regimens have the widest use. Colorectal cancer (CRC) is the third most common cancer in men (10% of the total) and the second in women (9.2%), with over 1.3 Million cases (746 000 men and 614 000 women) reported worldwide during 2012. The geographic incidence of CRC varies widely across the world, and the geographical patterns are very similar in men and women. Incidence rates vary ten-fold in both sexes worldwide, the highest estimated rates being in Australia/New Zealand (ASR 44.8 and 32.2 per 100,000 in men and women respectively), and the lowest in Western Africa (4.5 and 3.8 per 100,000). The incidence increases with age and is highest amongst the elder population, i.e. 60-64 years: 67.4; 65-69 years: 95.1; 70-74 years: 127.8; and? 75 years: 196.2 per 100 000 [Ferlay J, Soerjomataram I, Ervik M, Dikshit R, Eser S, Mathers C, Rebelo M, Parkin D M, Forman D, Bray, F. GLOBOCAN 2012 v1.0, Cancer Incidence and Mortality Worldwide: IARC CancerBase No. 11. Lyon, France: International Agency for Research on Cancer; 2013].
Approximately 40-50% of the affected patients develop metastatic disease and more than half a million deaths are reported annually as a consequence of CRC [Jemal A, Bray F, Center M M, Ferlay J, Ward E, Forman D. Global cancer statistics. CA Cancer J Clin. 2011 March-April; 61(2):69-90]. Indeed CRC accounted for 694 000 deaths worldwide solely during 2012 (8.5% of the total) [Ferlay J, Soerjomataram I, Ervik M, Dikshit R, Eser S, Mathers C, Rebelo M, Parkin D M, Forman D, Bray, F. GLOBOCAN 2012 v1.0, Cancer Incidence and Mortality Worldwide: IARC CancerBase No. 11 [Internet]. Lyon, France: International Agency for Research on Cancer; 2013].
CRC patients are usually treated surgically and, in most circumstances, with curative intent. Surgery, in fact, remains the primary modality of treatment for malignancies of the lower gastrointestinal tract, and standard resection is the only therapy required for early-stage cancer [Nelson H, Petrelli N, Carlin A, Couture J, Fleshman J, Guillem J, et al. Guidelines 2000 for colon and rectal cancer surgery. J Natl Cancer Inst. 2001 Apr. 18; 93(8):583-96]. As the stage of the tumor advances, in terms of depth of penetration and lymph node involvement, the chance of cure with surgery alone diminishes and the rate of local recurrence increase. In such cases, surgery may either be combined with adjuvant treatment or be performed for palliative control of symptoms only.
Adjuvant therapies have been shown to improve treatment outcome in metastatic CRC with prolonged survival [Cunningham D, Atkin W, Lenz H J, Lynch H T, Minsky B, Nordlinger B, et al. Colorectal cancer. Lancet. 2010 Mar. 20; 375(9719):1030-47]. Standard first-line adjuvant therapy of CRC includes single and combination chemotherapy with the agent 5-Fluorouracil (5-FU) [Cunningham D (2010)]. Treatment with 5-FU is usually given in combination with high doses of folate (or Leucovorin, LV) which significantly enhances the therapeutic effect of 5-FU in metastatic colorectal carcinoma. In fact, modulation of 5-FU with LV in metastatic disease has shown prolongation of the time-to-progression (TTP) of disease [Petrelli N, Douglass H O Jr, Herrera L, Russell D, Stablein D M, Bruckner H W, et al. The modulation of fluorouracil with leucovorin in metastatic colorectal carcinoma: a prospective randomized phase III trial. Gastrointestinal Tumor Study Group. J Clin Oncol. 1989 Oct; 7(10):1419-26].
For colorectal tumors, the original response rate for 5-FU given as a monotherapy was only around 10%. By adding Leucovorin (LV) the response rate was improved to 21% [Thirion P, Michiels S, Pignon J P, Buyse M, Braud A C, Carlson R W, O'Connell M, Sargent P, Piedbois P (2004) Modulation of fluorouracil by leucovorin in patients with advanced colorectal cancer: an updated meta-analysis. J Clin Oncol 22(18):3766-3775]. However, LV needs to be converted to the active metabolite [6R]-5,10-methylenetetrahydrofolate (methyleneTHF), which subsequently forms a ternary complex with deoxyuridine monophosphate (dUMP) and the target enzyme thymidylate synthase (TS) in a reaction where dUMP is converted to dTMP [Jarmula A, Cieplak P, Montfort W R (2005) 5,10-Methylene-5,6,7,8-tetrahydrofolate conformational transitions upon binding to thymidylate synthase: molecular mechanics and continuum solvent studies. J Comput Aided Mol Des 19(2):123-136]. This reaction is inhibited when the fluorinated metabolite of 5-FU, FdUMP, binds the complex instead of dUMP [Parker W B, Cheng Y C (1990) Metabolism and mechanism of action of 5-fluorouracil. Pharmacol Ther 48(3):381-395]. As such, LV does not have antitumoral effect, but enhances the effect of 5-FU by providing methyleneTHF in abundance, which stabilizes the ternary complex [Porcelli L, Assaraf Y G, Azzariti A, Paradiso A, Jansen G, Peters G J (2011) The impact of folate status on the efficacy of colorectal cancer treatment. Curr Drug Metab 12(10):975-984]. The inhibition impacts cells with a high proliferation rate most, such as tumor epithelial cells. This in turn leads to suppression of DNA synthesis in the cells, which may lead to cell death by apoptosis.
The required metabolic activation of LV into methyleneTHF is likely to lead to interindividual differences, which may be the reason the response rate for 5-FU given as a monotherapy was only improved to 21%.
A reduced folate, fotrexorin calcium (CoFactor®) ((dl)-5,10,-methylenepteroyl-monoglutamate calcium salt, or [6R,S]-5,10-methylene-THF Ca salt), also known as racemic methyleneTHF, has been suggested as an alternative to LV based on the assumption that direct administration of the reduced folate methyleneTHF in place of LV might offer significant advantages with respect to clinical activity. CoFactor® is a 1:1 mixture of the two diastereoisomers [Odin, E., Carlsson, G., Frosing, R., Gustaysson, B., Spears, C. P., Larsson, P. A., 1998. Chemical stability and human plasma pharmacokinetics of reduced folates. Cancer Invest. 16, 447-455]. As the [6R]-isomer is the directly active co-substrate of TS, it was anticipated that the administration of CoFactor®, instead of leucovorin, would be advantageous due to lower inter- and intrapatient variability regarding both clinical safety and efficacy.
Indeed, in a Phase II Trial in previously untreated metastatic colorectal cancer, the response rate for CoFactor® was found to be 35% [ Saif, M. W, Merritt, J, Robbins J, Stewart J., Schupp, J, 2006. Phase III Multicenter Randomized Clinical Trial to Evaluate the Safety and Efficacy of CoFactor®/5-Fluorouracil/Bevacizumab Versus Leucovorin/5-Fluorouracil/Bevacizumab as Initial Treatment for Metastatic Colorectal Carcinoma Clinical Colorectal Cancer, Vol. 6, No. 3, 229-234, 2006], and in another phase I/II clinical trial it was demonstrated that CoFactor® combined with 5-FU showed clinical benefit in pancreas cancer, defined as stable disease or tumor response, in 40% of patients [ Saif, M. W., Makrilia N., Syrigos K., 2010. CoFactor: Folate Requirement for Optimization of 5-Fluouracil Activity in Anticancer Chemotherapy. Journal of Oncology Vol. 1-5]. However, apart from presenting an unnecessary hepatic detoxification burden, the unnatural (6S)-isomer is a partial competitive inhibitor of the natural [6R]-isomer regarding its effect as co-substrate for TS [Leary, R. P., Gaumont, Y., Kisliuk, R. L., 1974. Effects of the diastereoisomers of methylenetetrahydrofolate on the reaction catalyzed by thymidylate synthetase. Biochem. Biophys. Res. Commun. 56, 484-488]. Furthermore, in a Phase IIb study CoFactor® in colorectal cancer was not demonstrated to be more efficacious than leucovorin as no significant differences between the study arms with regard to either efficacy or safety could be found, and a planned Phase III study colorectal cancer was discontinued before completion [Press release: ADVENTRX Provides Update on Cofactor Program. Nov. 2, 2007]. There thus remains a need for an improved folate-enhanced 5-FU treatment protocol by which the ternary complex is stabilized and the inhibition of TS is enhanced above the level currently achieveable with leucovorin.